Cellulose ether-polyacrylamide aqueous gels

ABSTRACT

Improvements in secondary recovery operations for the recovery of oil, and improvements in well-drilling operations, are accomplished through the use of aqueous gels exhibiting high gel strength prepared from water and a mixture of (a) at least one cellulose ether, e.g., carboxymethyl cellulose, and (b) at least one polyacrylamide.

This application is a division of our copending application having Ser.No. 611,751, filed Sept. 10, 1975, now U.S. Pat. No. 3,971,440.

This invention relates to new aqueous gels exhibiting improved physicalproperties and uses thereof, e.g., the plugging or sealing of fracturesin formations.

The secondary recovery of oil from oil-bearing subterranean formationsby fluid drive processes wherein a fluid is injected into the formationvia one or more injection wells to drive the oil through the formationto one or more production wells is a well-known process. Fluids used insuch processes include liquids such as water and various hydrocarbonsand gases such as steam, hydrocarbon gases, carbon dioxide, etc. Manyoil reservoirs comprise layers or zones of porous rock which can vary inpermeability from more than 1,000 millidarcys to less than 10millidarcys. In all fluid drive processes, a recognized problem is thepredilection of the drive fluid to channel along or through the morepermeable zones of the formation. This is commonly referred to asfingering. The more conductive zones, after the oil has been largelydisplaced therefrom, function as "thief zones" which permit the drivefluid to channel directly from injection to production wells. In manyinstances, such channeling or fingering results in leaving substantialquantities of oil in the less permeable zones of the formation which arebypassed. Such channeling or fingering can occur when the mobility,i.e., the quotient of the reservoir's permeability to the drive fluiddivided by the viscosity of the drive fluid, becomes large relative tothe mobility of the reservoir oil.

Drilling fluids used in the drilling of oil wells, gas wells, andsimilar boreholes are commonly aqueous liquids containing clays or othercolloidal materials. The drilling fluid serves as a lubricant for thebit and drill stem, as a carrying medium for the cuttings produced bythe drill bit, and assists in the formation of a filter cake on the wallof the borehole for the reduction of fluid losses to the surroundingsubsurface strata. Generaly, drilling fluids require the presence ofagents which increase the viscosity and gel strength of the fluid inorder that it may carry out these functions.

The present invention provides a solution for the above-describedproblems and other problems related thereto. We have now discovered aclass of new aqueous gels having an excellent balance of physicalproperties which can comprise at least a portion of the aqueous mediumused in secondary recovery operations, in fracturing fluids, and theaqueous medium used in well-drilling operations.

According to the invention, we have found that water-soluble orwater-dispersible mixtures of cellulose ethers and polyacrylamides, whenused in combination with a water-soluble compound of a polyvalent metalwhich can be reduced to a lower valence state, and a suitable reducingagent capable of reducing said polyvalent metal to said lower valencestate, can be used as gelling agents to gel aqueous mediums comprisingwater. We have discovered that by varying the composition and/or amountsof said gelling agents, and/or the conditions under which they are usedin forming the gels, a wide range of aqueous gels ranging from liquidhighly mobile gels to rigid gels exhibiting high gel strength can beproduced.

The aqueous gels of the invention are particularly useful in operationswherein a fluid medium is introduced into a borehole in the earth, e.g.,in the above-described secondary recovery operations, in theabove-described well drilling operations, in well completion operations,as fracturing fluids, packer fluids, etc.

The term "polymer," as used herein, is employed generically to includeboth homopolymers and copolymers, and the term "water-dispersiblepolymers," as used herein, is employed to include those polymers whichare truly water-soluble and those which are dispersible in water or theaqueous medium to form stable colloidal suspensions which may becrosslinked as described herein.

This invention resides in a method wherein a fluid medium is introducedinto a borehole in the earth and into contact with a subterraneanformation penetrated by said borehole. At least a portion of said fluidmedium comprises an aqueous gel, and said gel comprises water to whichthere has been added: a water-thickening amount of a water-solublemixture of (a) at least one cellulose ether and (b) at least onepolyacrylamide; a sensible amount of a water-soluble compound of apolyvalent metal wheren the metal present is capable of being reduced toa lower polyvalent valence state and which is sufficient to gel waterwhen the valence of at least a portion of the metal is reduced to alower valence state; and an amount of a water-soluble reducing agentwhich is effective to reduce at least a portion of the metal to a lowervalence state.

In a preferred embodiment, according to the invention, an aqueous gel isprepared from a polymeric mixture of (a) at least one water-solublecellulose ether, e.g., carboxymethyl cellulose, and (b) at least onewater-soluble polyacrylamide wherein the amount of cellulose etherpresent in the mixture of polymers ranges from about 40 to about 90weight percent, based on total polymers present, with the balance of theaqueous gel being water, a water-soluble compound of a polyvalent metaloxidizing agent, and a water-soluble reducing agent.

If desired, filler materials such as silica flour, diatomaceous earth,ground nutshells, finely divided natural sands, clays or clay-likematerials such as Illite clay and kaolin, and finely divided plasticparticles such as Microethene, etc., can be incorporated in the aqueousgels used in the practice of the invention.

In general, any of the water-soluble cellulose ethers can be used in thepractice of the invention. Cellulose ethers which can be used include,among others: the various carboxyalkyl cellulose ethers, e.g.,carboxyethyl cellulose and carboxymethyl cellulose (CMC); mixed etherssuch as carboxyalkyl hydroxyalkyl ethers, e.g., carboxymethylhydroxyethyl cellulose (CMHEC); hydroxyalkyl celluloses such ashydroxyethyl cellulose, and hydroxypropyl cellulose; alkylhydroxyalkylcelluloses such as methylhydroxypropyl cellulose; alkylcarboxyalkylcelluloses such as ethylcarboxymethyl cellulose; and hydroxyalkylalkylcelluloses such as hydroxypropylmethyl cellulose; and the like. Many ofthe cellulose ethers are available commercially in various grades.

The carboxy-substituted cellulose ethers are available as the alkalimetal salt, usually the sodium salt. However, the alkali metal is seldomreferred to, and they are commonly referred to as CMC, CMHEC, etc. Forexample, water-soluble CMC is available in various degrees ofcarboxylate substitution ranging from about 0.3 up to 3.0. In general,CMC having a degree of substitution in the range of 0.65 to 0.95 ispreferred. Frequently, CMC having a degree of substitution in the rangeof 0.85 to 0.95 is a more preferred cellulose ether. CMC having a degreeof substitution less than the above-preferred ranges is usually lessuniform in properties and thus less desirable for use in the practice ofthe invention. CMC having a degree of substitution greater than theabove-preferred ranges usually has a lower viscosity and more isrequired in the practice of the invention. The degree of substitution ofCMC is commonly designated in practice as CMC-7, CMC-9, CMC-12, etc.,where the 7, 9, and 12 refer to a degree of substitution of 0.7, 0.9,and 1.2, respectively.

In the above-described mixed ethers, it is preferred that the portionthereof which contains the carboxylate groups be substantial instead ofa mere trace. For example, the CMHEC it is preferred that thecarboxymethyl degree of substitution be at least 0.4. The degree ofhydroxyethyl substitution is less important and can vary widely, e.g.,from about 0.1 or lower to about 0.4 or higher.

In general, the polyacrylamides used to carry out this invention have amolecular weight of about 200,000 to about 16 million with from 2percent to about 75 percent, preferably from about 3 to about 40percent, of the amide groups being hydrolyzed to carboxyl groups. Thehydrolysis of acrylamide polymer is accomplished by reacting the samewith sufficient aqueous alkali, e.g., sodium hydroxide, to hydrolyzebetween about 75 percent of the amide groups present in the polymermolecule. The resultant product comprises a long carbon chain, toalternate carbon atoms of which there are attached either amide orcarboxyl groups. Polymers in which less than about 14 percent of theamide groups have been hydrolyzed are considered to be more useful withhigh salinity waters than those polymers having a higher degree ofhydrolysis.

Polymers of acrylamide which contain the above-described amounts ofcarboxyl groups can, alternatively, be prepred by copolymerization of amixture of acrylamide and acrylic acid.

Small amounts of the mixture of polymers will usually produce liquidmobile gels which can be readily pumped whereas large amounts of themixture will usually produce stiff, rigid gels. If desired, said stiffgels can be "thinned" by dilution to any desired concentration of themixture of cellulose ether and polyacrylamide. This can be done bymechanical means, e.g., stirring, pumping, or by means of a suitableturbulence-inducing device, such as jet nozzle.

Crosslinking agents which can be used in the practice of the inventionare water-soluble compounds of polyvalent metals wherein the metal ispresent in a valence state which is capable of being reduced to a lowervalence state. Examples of such compounds include potassiumpermanganate, sodium permanganate, ammonium chromate, ammoniumdichromate, the alkali metal chromates, the alkali metal dichromates,and chromium trioxide. Sodium dichromate and potassium dichromate,because of low cost and ready availability, are the presently preferredmetal-containing compounds for use in the practice of the invention. Thehexavalent chromium in said chromium compounds is reduced in situ totrivalent chromium by suitable reducing agents as discussed hereinafter.In the permanganate compounds the manganese is reduced from +7 valenceto +4 valence as in MnO₂.

The amount of metal-containing compounds used in the practice of theinvention will be a sensible amount, i.e., a small but finite amountwhich is more than incidental impurities, but which is effective orsufficient to cause subsequent gelation when the metal in the polyvalentmetal compound is reduced to a lower valence state. The lower limit ofthe concentration of the starting metal-containing compound will dependupon several factors including the particular type of cellulose etherand polyacrylamide mixture used, the concentration of said mixture inthe water to be gelled, the water which is used, and the type of gelproduct desired. For similar reasons, the upper limit on theconcentration of the starting metal-containing compound also cannotalways be precisely defined. As a general guide, the amount of thestarting polyvalent metal-containing compound used in preparing aqueousgels in accordance with the invention will be in the range of from 0.05to 30, preferably 0.5 to 30, weight percent of the amount of the totalpolymer mixture used. Those skilled in the art can determine the amountof starting polyvalent metal-containing compound to be used by simpleexperiments carried out in the light of this disclosure. For example, wehave discovered that when brines such as are commonly available inproducing oil fields are used as the water in preparing gels inaccordance with the invention, less of the starting polyvalentmetal-containing compound is required than when distilled water is used.Gelation rates are frequently faster when using brines. Such oil fieldbrines commonly contain varying amounts of sodium chloride, calciumchloride, magnesium chloride, etc. Sodium chloride is usually present inthe greatest concentration. The word "water" is used generically hereinand in the claims, unless otherwise specified, to include such brines,fresh water, and other aqueous media which can be gelled in accordancewith the invention.

Suitable reducing agents which can be used in the practice of theinvention include sulfur-containing compounds such as sodium sulfite,sodium hydrosulfite, sodium metabisulfite, potassium sulfite, sodumbisulfite, potassium metabisulfite, sodium sulfide, sodium thiosulfate,ferrous sulfate, thioacetamide, and others; and nonsulfur-containingcompounds such as hydroquinone, ferrous chloride, p-hydrazinobenzoicacid, hydrazine phosphite, hydrazine dichloride, and others. Some of theabove reducing agents act more quickly than others, for example, sodiumthiosulfate usually reacts slowly in the absence of heat, e.g.,requiring heating to about 50° C. The presently most preferred reducingagents are sodium hydrosulfite or sodium bisulfite.

The amount of reducing agent to be used in the practice of the inventionwill be a sensible amount, i.e., a small but finite amount which is morethan incidental impurities, but which is effective or sufficient toreduce at least a portion of the higher valence metal in the startingpolyvalent metal-containing compound to a lower valence state. Thus, theamount of reducing agent to be used depends, to some extent at least,upon the amount of the starting polyvalent metal-containing compoundwhich is used. In many instances, it will be preferred to use an excessof reducing agent to compensate for dissolved oxygen in the water,exposure to air during preparation of the gels, and possible contactwith other oxidizing substances such as might be encountered in fieldoperations. As a general guide, the amount of reducing agent used willgenerally be at least 150, preferably at least about 200, weight percentof the stoichiometric amount required to reduce the metal in thestarting polyvalent to said lower valence state, e.g., +6 Cr to +3 Cr.Those skilled in the art can determine the amount of reducing agent tobe used by simple experiments carried out in the light of thisdisclosure.

The mixture of at least one cellulose ether and at least onepolyacrylamide is necessary to form the gels of this invention. In thepolymeric component mixture of this invention, the cellulose ethercomponent of the mixture comprises in the range of about 25 to about 95percent by weight, preferably about 40 to about 90 percent by weight,and more preferably about 60 to about 80 weight percent, of thepolymeric component mixture. The polymeric mixture will comprise in therange of 0.0025 to 5 weight percent, preferably 0.025 to 2 weightpercent, of the aqueous gel, based on the weight of water.

The proportions of the two polymer types which provide a gel strengthimprovement will depend somewhat on the total polymer concentration inthe aqueous gel. For example, at a 0.25 weight percent (2500 ppm) totalpolymer level, substantial improvements in gel strengths have beenobserved when 60-90 weight percent of the polymeric component was asoluble cellulose ether. At a one weight percent (10,000 ppm) totalpolymer level, gel strength improvements were observed when a celluloseether represented 30-80 weight percent of the polymeric components. Inany event, gel strength-improving proportions of the two polymer typeswill be used in the improved compositions and processes of the presentinvention.

Various methods can be used for preparing the aqueous gels of theinvention. Either the polyvalent metal-containing compound of thereducing agent can be first added to a solution of the polymeric mixtureof at least one cellulose ether and at least one polyacrylamide in wateror other aqueous medium, or the metal-containing compound and thereducing agent can be added simultaneously to a solution or an aqueousmedium containing the polymeric mixture. Generally speaking, whereconvenient, the preferred method is to first disperse the polymermixture in the water or other aqueous medium. The reducing agent is thenadded to the dispersion with stirring. The metal-containing compound isthen added to the solution or aqueous medium containing the celluloseether and polyacrylamide and the reducing agent, with stirring. Gelationstarts as soon as reduction of some of the higher valence metal in thestarting polyvalent metal-containing compound to a lower valence stateoccurs. The newly formed lower valence metal ions, for example, +3chromium obtained from +6 chromium, effect rapid crosslinking of thepolymer mixture and gelation of the solution or aqueous mediumcontaining same.

One presently preferred method of preparing the aqueous gels is toprepare the gel while the components thereof are being pumped into thewell. This method comprises preparing a base solution of the celluloseether and polyacrylamide, adding to this base solution (a) a polyvalentmetal compound such as sodium dichromate or (b) a reducing agent such assodium thiosulfate or sodium bisulfite, pumping the base solution downthe well and into the fracture, and during pumping adding to said basesolution the other of the reagents (a) and (b) which was not previouslyadded thereto. It is also within the scope of the invention toincorporate all the components of the aqueous gel into a stream of waterwhile it is being pumped, e.g., into a well. For example, CMC andpolyacrylamide can be added first to the flowing stream of water and theother components added subsequently in any suitable order. Turbulentflow conditions in the pipe will provide proper mixing.

It is also within the scope of the invention to prepare a dry mixture ofthe cellulose ether-acrylamide polymers, the metal-containing compound,and the reducing agent, in proper proportions, and then add this drymixture to the proper amount of water.

An advantage of the invention is that ordinary ambient temperatures andother conditions can be used in practically all instances in preparingthe aqueous gels of the invention or aqueous medium containing same.However, in some instances, a small amount of heat may be desirable toaid in the formation of the gel, e.g., heating to a temperature of about50° C.

One procedure which can be used to prepare the gels is to prepare arelatively concentrated or high viscosity gel and dilute same to aviscosity or concentration suited for the actual use of the gel. In manyinstances, this procedure results in a more stable gel.

Aqueous gels in accordance with the invention can be prepared having awide range of viscosities or firmness ranging from low viscosity orhighly mobile gels having a relatively low viscosity up to firm or rigidgels which are nonmobile. The choice of gel viscosity or concentrationwill depend upon the use to be made of the gel. The actual viscosityand/or gel strength of the gel will depend upon the type andconcentration of cellulose ether-acrylamide polymer mixture, the typeand amount of starting polyvalent metal compound used and the type andamount of reducing agent used.

Herein and in the claims, unless otherwise specified, the aqueous gelsof the invention are defined for convenience, and not by way oflimitation, in terms of the amount of the mixture of cellulose ether andpolyacrylamide contained therein, irrespective of whether or not all themixture is crosslinked. For example, a one weight percent or 10,000 ppmgel is a gel which was prepared from a starting mixture solution whichcontained one weight percent or 10,000 ppm by weight of the mixture ofcellulose ether and polyacrylamide. The same system is employed for thegels prepared by the above-described dilution technique.

As indicated above, the aqueous gels of the invention are particularlyuseful in fluid drive operations for the secondary recovery of oil. Thegels of the invention are applicable to decreasing the mobility of adrive fluid, such as water, or decreasing the permeability of formationsprior to or during secondary recovery operations, such as fluid driveprocesses, and also for water shutoff treatments in producing wells. Inone embodiment of the invention, a conventional waterflood or gas driveis carried out in conventional manner until the drive fluid breaksthrough into the production well in excessive amounts. A gel of theinvention is then pumped down the well and into the formation in anysuitable manner, any suitable amount, and for any desired period of timesufficient to obtain the desired indepth penetration and decrease inmobility of the drive fluid, or decrease in permeability of the highpermeability zones of the formation. Usually, an in-depth penetration offrom 10 to 1,000, preferably 75 to 900, feet from the injection wellwill be sufficient. However, this can vary from formation to formation,and penetrations outside said ranges can be used. For example, there canbe injected into the formation via the injection well from about 0.001to about 0.5 pore volume of a gel in accordance with the invention overa suitable period of time ranging from one day to six months; or theinjection of the gel can be carried out by injecting a slug of about 200to 5,000 barrels of gel into the well and then into the formation.Injection in one of the above manners will provide a flood frontadjacent the oil to be produced. If desired, an ordinary brine or watercan then be employed to drive this slug or band or front of gel onthrough the formation to the production well. If desired, in order toavoid any sharp demarcations in viscosity or mobility of the gel, whichcould adversely affect the relative mobility of the flood medium and theoil and cause channeling, the viscosity or concentration of the gel cangradually be lessened through a series of incremental decreases ratherthan discontinuing the injection thereof abruptly.

In another embodiment of the invention, the formation can be treatedprior to carrying out the fluid drive secondary recovery operations.This embodiment is particularly applicable where there is good knowledgeof the nature of the formation. Thus, in a formation where theoil-bearing strata are interspersed with more permeable porous stratawhich contain no oil or an insufficient amount of oil to make secondaryrecovery operations economical, but which more permeable strata wouldstill act as a thief zone, the formation can be treated in accordancewith the invention prior to initiating the fluid drive operation.

In still another embodiment, the invention can be applied to producingwells, either oil wells or gas wells, where there is a more porousnonhydrocabon-bearing strata adjacent the hydrocarbon-bearing strata.For example, such a condition can exist where there is a water sandadjacent the hydrocarbon-bearing sand and the water intrudes into theborehole and interferes with the production of hydrocarbon. In suchinstances, the formation can be treated in accordance with the inventionto shut off the flow of water. The method of carrying out such a watershutoff treatment is substantially the same as described above inconnection with fluid drive operations.

It is also within the scope of the invention to carry out the gelinjection techniques of the invention periodically or intermittently, asneeded, during the course of a fluid drive secondary operation, orduring the production of oil from a producing well.

In all of the above operations, the injection of the gel of theinvention can be carried out in conventional manner. If desired, a gelof suitable viscosity or concentration can be injected as the drivefluid per se. Gels injected in accordance with the invention can beprepared in advance, stored in suitable tanks, and then pumped into thewell; or said gels can be formed in a conduit leading to the injectionwell, or in the tubing in the well itself, and then injected into theformation. Thus, the required amounts of the mixture of celluloseether-polyacrylamide, polyvalent metal compound, and reducing agent canbe metered into the tubing in the well, mixed therein, and then injectedinto the formation. If desired, selected portions of the formation canbe isolated mechanically, as by the use of packers, and other meansknown to the art, for treatment in accordance with the invention.

The aqueous gels of the invention can comprise, or can be employed as,drilling fluids in the drilling of wells in any manner known to the artfor the use of drilling fluids. Such gels can be employed without theaddition of other materials thereto. However, if desired, weightingagents such as barium carbonate, barium sulfate, amorphous silica, etc.,can be included in the drilling fluids comprising said aqueous gels. Ifdesired, other additives compatible with the aqueous gels can also beincluded in the drilling fluid. Thus, the drilling fluids can includeclays such as bentonite, attapulgus clay, fluid loss agents, etc. Itshould be understood that not all of these additives or constituentswill necessarily be present in any one drilling fluid and that theamount of any particular additive used will be governed by the otherconstituents present under the particular well conditions existing. Asindicated, in selecting such additives for use in a particular drillingfluid, care should be taken to avoid materials which are not compatiblewith the aqueous gels. The use of such additives will be governed inpart by the viscosity and fluid loss properties desired in the drillingfluid. Thus, as is the situation in connection with conventionaldrilling fluids, pilot tests should be run to determine the propertiesdesired for the aqueous gel used as the drilling fluid, or an aqueousgel containing one of the above-described additives, to determine theoptimum results or properties desired for the drilling fluid under theparticular well conditions existing.

The following examples will serve to further illustrate the invention.

EXAMPLE

A series of runs was made to illustrate the formation of aqueous gels inaccordance with the invention and to demonstrate the increase in gelstrength obtained with the inventive mixtures of cellulose ethers andacrylamide polymers. In the series of runs, aqueous gels were preparedwith different concentrations of the individual polymers and mixtures ofthe polymers. Specifically, gels were made with total polymerconcentrations of 2,500 ppm, 5,000 ppm, and 10,000 ppm. In each seriesof runs the amount of each polymer in the mixtures was varied by 10percent ranging from 0 to 100 percent for each.

The individual solutions were prepared by adding varying amounts of thepolymers to water and adding varying amounts of NaHSO₃ and varyingamounts of Na₂ Cr₂ O₇.2H₂ O, both of which were dissolved in water. Themixtures were stirred and allowed to form gels. Stable gels were formedin each instance.

In the first series of runs, 2,500 ppm of polymer, either polyacrylamidealone or carboxymethyl cellulose alone or mixtures thereof, were gelledwith sodium bisulfite and sodium dichromate. The amount of sodiumdichromate used was 500 ppm and the amount of sodium bisulfite used was750 ppm. The gel strength for the various gels prepared was evaluated.

The results for this series of runs are set forth below in Table I.

                  TABLE I                                                         ______________________________________                                        GELS FROM MIXTURES OF POLYACRYLAMIDE                                          AND CARBOXYMETHYL CELLULOSE                                                   AT 2,500 PPM TOTAL POLYMER                                                    Solution CMC      Polyacrylamide                                                                             Gel Strength                                   No.      (wt. %)  (wt. %)      (lb/100 ft..sup.2)*                            ______________________________________                                        1        100       0           1201                                           2        90       10           3242                                           3        80       20           900                                            4        70       30           1661                                           5        60       40           1801                                           6        50       50           600                                            7        40       60           380                                            8        30       70           110                                            9        15       85           6.67                                           10        0       100          6.67                                           ______________________________________                                         *Gels were aged 18 hours at room temperature.                            

The above data in general demonstrate that polymer mixtures (2500 ppmtotal polymer) containing in the range of about 60 to about 90 weightpercent of a cellulose ether such as carboxymethyl cellulose (CMC) and40 to 10 weight percent polyacrylamide gives gels with greater gelstrengths than the gels obtained from solutions of CMC alone orpolyacrylamide alone.

In another series of runs 5,000 ppm of total polymer, eitherpolyacrylamide or carboxymethyl cellulose or mixtures thereof, wasgelled using 1,000 ppm sodium dichromate and 1,250 ppm sodium bisulfite.The gel strengths of these samples are given in Table II.

                  TABLE II                                                        ______________________________________                                        GELS FROM MIXTURES OF POLYACRYLAMIDE                                          AND CARBOXYMETHYL CELLULOSE                                                   AT 5,000 ppm TOTAL POLYMER                                                    Solution                                                                              CMC       Polyacrylamide                                                                             Gel Strength                                   No.     (wt. %)   (wt. %)      (lb/100 ft..sup.2)*                            ______________________________________                                        1       100        0           2550                                           2       90        10           3100                                           3       80        20           4600                                           4       70        30           4500                                           5       60        40           5100                                           6       50        50           3200                                           7       40        60           1200                                           8       30        70           923                                            9       20        80           848                                            10      10        90           193                                            11       0        100          43.5                                           ______________________________________                                         *Gels were aged 18 hours at room temperature.                            

The above data demonstrate that polymer mixtures (5,000 ppm totalpolymer) containing in the range of about 50 to about 80 weight percentof a cellulose ether such as carboxymethyl cellulose (CMC) and 50 to 20weight percent polyacrylamide give gels with greater gel strengths thanthe gels obtained from solutions of CMC alone or polyacrylamide alone.

In another series of runs, 10,000 ppm of total polymer, eitherpolyacrylamide or carboxymethyl cellulose or mixtures thereof, was usedto form gels using 1,500 ppm sodium dichromate and 2,000 ppm sodiumbisulfite. The gel strengths of these samples are given in Table III.

                  TABLE III                                                       ______________________________________                                        GELS FROM MIXTURES OF POLYACRYLAMIDE                                          AND CARBOXYMETHYL CELLULOSE                                                   AT 10,000 ppm TOTAL POLYMER                                                   Solution                                                                              CMC       Polyacrylamide                                                                             Gel Strength                                   No.     (wt. %)   (wt. %)      (lb/100 ft..sup.2)*                            ______________________________________                                        1       100       `0            6470                                          2       90        10            6830                                          3       80        20           10,672                                         4       70        30           15,808                                         5       60        40           13,674                                         6       50        50           14,000                                         7       40        60           12,006                                         8       30        70           10,672                                         9       15        85           10,000                                         10       0        100           9,338                                         ______________________________________                                         *Gels were aged 18 hours at room temperature.                            

The above data demonstrate that polymer mixtures (10,000 ppm totalpolymer) containing in the range of about 30 to about 80 weight percentof a cellulose ether such as carboxymethyl cellulose (CMC) and 70 to 20weight percent polyacrylamide give gels with greater gel strengths thanthe gels obtained from solutions of CMC alone or polyacrylamide alone.

The CMC used in the runs of Tables I-III was sodium carboxymethylcellulose (Hercules CMC-9H) having a degree of substitution of about0.9.

The polyacrylamide used in the runs of Tables I-III was Polyfloc Hi-Visobtained from Betz Laboratories, Inc. having a degree of hydrolysis ofabout 27-32 percent.

The runs in Tables I-III illustrate the unexpectedly high gel strengthobtained with a gelled mixture of polyacrylamide and carboxymethylcellulose especially when the amount of carboxymethyl cellulose presentin the polymer mixture ranges from 40 to 90 weight percent.

The apparatus employed in making gel strength determinations comprises amodification of a standard Model 35 Fann V-G meter manufactured byGeophysical Machine Works, Houston, Tex. In modifying said Fann V-Gmeter, the conventional viscosity measuring cup, sleeve, and bob wereremoved. An adapter shaft was then removably attached at its upper endto the lower end of the regular torque shaft to which said viscositymeasuring bob is normally attached. The upper end of said torque shaftis not altered and is connected in its normal manner to the torquemeasuring components of the Fann V-G meter. A cylindrical shear bobhaving a diameter of 1/2 inch, a length of 11/2 inches, and having aspindle shaft extending therethrough was provided. The surface of saidshear bob is roughened, knurled, or etched for purposes describedhereinafter. Said spindle shaft was adapted at its upper end to beremovably attached to the lower end of said adapter shaft. The lower endof said spindle shaft extends through said shear bob for a purposedescribed hereinafter. A vertically adjustable sample holder comprisinga platform mounted on the shaft of a motor having a speed of rotation of1 rpm was positioned below said regular torque shaft.

In preparing for a test, the roughened surface of the shear bob iscovered with a thin absorbent paper fixed to said surface with a thincoat of rubber cement. A steel spacer element having a hole in thecenter thereof is placed in the bottom of a 100 ml beaker. Next,approximately 85 to 90 ml of the newly mixed gel to be tested and whichwas prepared as described above is placed in said beaker. Thepaper-covered shear bob is then placed in the gel in the beaker with thelower end of said spindle shaft in the hole in said steel spacer. Aplastic spacer element having a hole in the center thereof is thenplaced over the upper end of said spindle shaft and floats on thesurface of said gel. The purpose of said spacer elements is to insurethat the shear bob is maintained centered in the gel while the gel isforming and aging. The gel strength can then be measured as describedbelow. However, in most instances, it is preferred to allow the gel toage for a specific period to develop gel strength. Any suitable agingperiod, depending upon the concentration and other properties of thegel, can be used. Usually, periods up to about 24 hours are sufficient.The gels of Tables I, II, and III were aged about 18 hours. During saidaging period, the paper on the surface of the shear bob absorbs thegelling solution as gelation occurs, with consequent formation of gelwithin the pores of the paper, and thus provides a firm bond between thegel and the shear bob.

After aging, the beaker containing the gel and the shear bob positionedtherein is firmly mounted on the rotatable platform by means of a pieceof carpet tape having adhesive on both sides thereof. The upper spacerelement is removed and the upper end of the shear bob spindle shaft isconnected to the lower end of the adapter shaft which is then connectedat its upper end to the lower end of the Fann meter torque shaft. Themotor which normally drives said sleeve, as in viscosity measurements,is not used in these tests. The 1 rpm motor driving the sample platformis then started and the torsion dial in the top of the Fann V-G meter isobserved. As the beaker is rotated, there is an increase in the readingsof said dial. The rotation of said beaker is a function of time. Forexample, after one-half minute the beaker will have rotated 180°. Theshear bob in the gel resists this rotation due to the gel strength ofthe gel. The dial in the top of the Fann meter measures this resistancein degrees of tension applied against the calibrated spring in the Fannmeter torque measuring mechanism. When the gel strength of the gel isexceeded and the gel ruptures, said dial reading immediately decreases.The maximum reading at this point is recorded and employed in thefollowing formula to calculate the gel strength:

    G.sub.s = 100 K.sub.s θ/r.sub.b A.sub.b

where

G_(s) is the gel strength, in lbs./100 ft.²,

K_(s) is torsional spring constant, in ft.lbs./degree (converted fromDyne cm/degree as furnished by spring manufacturer),

θ is a dial reading, in degrees,

r_(b) is the radius of the shear bob, in feet, and

A_(b) is the area of the shear bob, in feet².

Since the curved surface of the cylindrical shear bob is covered with anabsorbent paper, the area of the ends of the shear bob is ignored inmaking calculations. A range of springs having different constants(stiffness) is available commercially for the Fan V-G meter. Byemploying springs having different degrees of stiffness, one can measuregel strengths over a range of from about 5 to 20,000 lbs./100 ft². Onthe modified Fann Viscometer it is easy to position an individual gelsample in the instrument and a measurement requires only one to twominutes to complete.

The tables show that a gel can be tailor-made from mixtures of celluloseethers and acrylamide polymers for a particular application with controlof gel strength for meeting the desired properties for the purposeintended.

Other modifications and alterations of this invention will becomeapparent to those skilled in the art from the foregoing discussion andexamples, and it should be understood that this invention is not to beunduly limited thereto.

We claim:
 1. A fluid-gelable composition consisting essentially of1. water;
 2. a water-thickening amount of a water-soluble polymeric mixture of (a) at least one carboxymethylcellulose ether and (b) at least one acrylamide homopolymer having a molecular weight of about 200,000 to about 16,000,000 with from 2 percent to about 75 percent of the amide groups being hydrolyzed to carboxyl groups wherein the amount of (a) present in the mixture ranges from about 25 to about 95 weight percent of the total amount of (a) plus (b);
 3. from 0.05 to about 60 weight percent of a water-soluble compound of a polyvalent metal wherein the metal present is capable of being reduced to a lower valence state and which is sufficient to gel said water when the valence of at least a portion of said metal is reduced to said lower valence state, said metal compound being selected from the group consisting of ammonium chromate, ammonium dichromate, the alkali metal chromates and dichromates, chromium trioxide, and mixtures thereof; and
 4. from 0.1 to at least about 200 percent of the stoichiometric amount of a water-soluble reducing agent which is effective to reduce at least a portion of said metal to said lower valence state, said reducing agent being selected from the group consisting of hydroquinone, sodium sulfide, sodium hydrosulfite, sodium metabisulfite, potassium sulfite, sodium bisulfite, potassium metabisulfite, sodium sulfite, sodium thiosulfate, ferrous sulfate, ferrous chloride, p-hydrazinobenzoic acid, hydrazine phosphite, hydrazine dihydrochloride, and mixtures thereof with the further proviso that the proportions of (a) and (b) in the water are such that the resulting gel strength of the composition is greater than that obtained with the same total polymer concentration of either (a) or (b) alone.
 2. A composition according to claim 1 wherein there is present in said composition2. from 0.0025 to about 5 weight percent of said polymeric mixture based upon the weight of said water and wherein the amount of (a) in the mixture ranges from about 40 to about 90 weight percent based upon the total amount of (a) plus (b).
 3. A composition according to claim 1 wherein (2) is a mixture of carboxymethylcellulose ether and a polyacrylamide having a degree of hydrolysis of about 27-32 percent.
 4. A composition according to claim 1 wherein there is present2. from 0.025 to about 2 weight percent of said polymeric mixture based upon the weight of said water;
 3. from 0.5 to about 30 weight percent of said polyvalent metal compound based upon the weight of said mixture; and
 4. from 0.5 to at least about 150 percent of the stoichiometric amount of said reducing agent required to reduce said polyvalent metal to said lower valence state.
 5. A composition according to claim 1 wherein2. is a mixture of carboxymethylcellulose ether and a polyacrylamide having a degree of hydrolysis of about 27-32 percent and the amount of (a) present in the mixture ranges from about 30 to about 80 weight percent of the total amount of (a) plus (b);
 3. is sodium dichromate; and
 4. is sodium bisulfite.
 6. A composition according to claim 1 wherein the amount of carboxymethylcellulose present in the polymeric mixture ranges from about 50 to about 80 weight percent based upon the total amount of (a) plus (b). 